Carbon fiber reinforced composite materials have excellent mechanical properties such as strength, toughness, durability and stiffness while being lightweight, and therefore have been widely deployed in aircraft structures, spacecraft structures, automobile structures, railway car structures, marine vessel structures, sporting goods, and computer such as housings for laptop computers. Therefore demand has been increasing each year in these fields. Aircraft structures and spacecraft structures in particular require excellent mechanical properties and heat resistance, and therefore carbon fibers are most commonly used as reinforcing fibers. Herein, spacecraft structures refer to structures that are used for example in man-made satellites, rockets, and space shuttles and the like.
Furthermore, the most common use of thermosetting resins as matrix resins are combinations of epoxy resins and polyamines that have excellent heat resistance, modulus of elasticity, and chemical resistance with minimal cure shrinkage. The manufacturing method of these carbon fiber reinforced composite materials primarily involves heating and pressure molding using an autoclave or the like, but there are problems with the high cost of molding, the large size of molding equipment, and restrictions in the molding size due to the equipment, and the like (for example, refer to patent document 1).
In light of the aforementioned problems, a low cost molding method has been proposed that can perform molding using only a vacuum pump and an oven, without using expensive molding equipment such as an autoclave. Conventionally, when molding a composite material by laminating a prepreg and curing, water vapor from the epoxy resin composition and air that are trapped in prepreg layers or between layers forms voids, and therefore high-pressure is often applied using an autoclave or the like during molding in order to prevent these voids from growing. However, recently, it has being reported that a low void panel could be achieved in a process that removes the water vapor of molding and trapped air to the outside of the molded panel, by using a partially impregnated prepreg where the matrix resin is partially impregnated into reinforcing fibers, and utilizing the unimpregnated section of the reinforcing fibers as an air path. However, when removing water vapor and trapped air by a vacuum pump, generally water vapor and trapped air must be removed outside of the molded panel by maintaining the prepreg at a relatively low temperature for a long period of time under vacuum to retain the air paths and the fluidity of the matrix resin, and therefore to effectively remove water vapor and trapped air, there is a problem that the molding time is much longer than with autoclave molding (for example, refer to patent document 2).
On the other hand, when using fiber reinforced composite materials in aircraft structural materials, enhancing the mechanical properties, particularly the compressive strength, is important, while at the same time enhancing properties such as heat resistance and environmental resistance is also important. A prepreg that uses a matrix resin containing tetraglycidyl amine type epoxy resin and diaminodiphenyl sulfone as a hardener provides high adhesion to reinforcing fibers and matrix resins, and the fiber reinforced composite material obtained has high mechanical properties. In addition, to achieve high quality of the cured parts for aircraft structures, conventionally an autoclave is used for these prepregs. However, the energy required for heating and pressuring is high when these prepregs are cured and formed into molded parts, at temperatures of approximately 180° C. and pressures of approximately 0.7 MPa. A large amount of energy is required for heating and pressurizing in autoclave processing. So there is strong demand for a prepreg that can be molded in a low cost process by energy savings and provide a fiber reinforced composite material that has high mechanical properties suitable for use in structural materials for aircraft, while simultaneously having high heat resistance and environmental resistance.
In the past, forming such prepreg systems which can be cured at low cost in a short period of time by low temperature cure using only a vacuum pump and an oven to achieve excellent mechanical properties, heat resistance, and environmental resistance suitable for structural materials for aircraft has been very difficult in the field of fiber reinforced composite materials.
In light of the foregoing, a partially impregnated prepreg has been proposed where the resin viscosity after heating and molding is controlled in consideration of the fluidity of the resin during the pressure reducing process. (For example, refer to patent document 3.) However, the resin viscosity is too high at room temperature so handling of the prepreg is difficult. In addition, diaminodiphenyl sulfone is not included in the epoxy resin composition, and the amount of tetraglycidyl amine type epoxy resin added is small, so the adhesion between the reinforcing fibers and the matrix resin and the environmental resistance such as the compressive strength at high temperatures after absorbing moisture are inferior, and thus are not suitable for use in structural materials of aircraft.
On the other hand, an epoxy resin composition containing dicyandiamide and a urea compound, as well as a prepreg thereof has been proposed in order to greatly reduce the energy consumption by low temperature cure of a prepreg that uses a matrix resin containing tetraglycidyl amine type epoxy resin and aminodiphenyl sulfone as a hardener (for example, refer to patent document 4 and patent document 5). However, US 2006-035088A1 introduces an epoxy resin composition and prepreg that uses a compression molding method and is specialized for general industry. This epoxy resin composition contains little tetraglycidyl amine type epoxy resin, which cannot provide strong adhesion between the reinforcing fibers and the matrix resin, and cannot provide mechanical properties and environmental resistance suitable for structural materials for aircraft. In addition, the molding method is compression molding that uses high temperatures, high pressures, and expensive equipment, so there are problems with reducing costs. In addition, the molding method uses high pressure, and there is no mention or suggestion of a method for reducing trapped air during lamination necessary for achieving low voids, nor of a method for restricting the generation of water vapor due to the epoxy resin composition, and a fiber reinforced composite material obtained by curing a prepreg containing this epoxy resin composition using only vacuum and an oven will have difficulty achieving low voids. JP2000-17090 discloses a prepreg specialized for autoclave molding at low temperature. However, in order to achieve low voids by curing with a vacuum pump and oven, both the trapped air during lamination must be reduced and handling of the prepreg must be seriously considered, and furthermore a specific room temperature viscosity range is necessary. Yet, none of these have been mentioned or discussed. In addition, the water vapor from the epoxy resin composition causes the formation of voids, and although there is a need to suppress the formation of water vapor and for the matrix resin to begin curing at a specific temperature range, there is no mention nor suggestion of this, and there is no mention whatsoever about a method for achieving low voids.
[Patent document 1] JP2004-050574A2
[Patent document 2] U.S. Pat. No. 6,391,436B1
[Patent document 3] JP2008-088276A2
[Patent document 4] US2006-0035088A1
[Patent document 5] JP2000-017090A2